Photolithography alignment mark structures, semiconductor structures, and fabrication method thereof

ABSTRACT

A method is provided for fabricating a photolithography alignment mark structure. The method includes providing a substrate; thrilling a first grating, a second grating, a third grating and a fourth grating in the substrate; forming a photoresist layer on a surface of the substrate; obtaining a first alignment center along a first direction and a second alignment center along a second direction based on the first grating and the fourth grating, respectively; providing a mask plate having a fifth grating pattern and a sixth grating pattern; aligning the mask plate with the substrate by using the first alignment center as an alignment center along the first direction and the second alignment center as an alignment center along the second direction; reproducing the fifth grating pattern and the sixth grating pattern in the photoresist layer: and forming a fifth grating and a sixth grating on the substrate by removing a portion of photoresist layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of Chinese patent application No.201410505490.0, filed on Sep. 26, 2014, the entirety of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to the field of semiconductormanufacturing technology and, more particularly, relates to aphotolithographic alignment mark structure and the fabricating processthereof.

BACKGROUND

With the development of semiconductor technology, semiconductor chiparea is getting smaller and smaller while the line width insidesemiconductor chip is also shrinking. Therefore, semiconductor processcapability is facing a growing challenge, the precision of the processand the control of process variations also became increasinglyimportant. Among the processes for fabricating semiconductor chips,photolithography technique is one of the most important processes.Photolithography is a technological process to transfer mask patterns ofa mask plate onto a wafer through a series of steps including alignment,exposure, development, etc. Therefore, the quality of thephotolithography process directly affects the performance of theultimately formed semiconductor chip.

During photolithography process, to accurately transfer a mask patternon a mask plate onto a wafer, a key step is to align the mask plate withthe wafer, that is, to calculate the position of the mask plate withrespect to the wafer to meet the requirements of registration accuracy.As the feature size getting smaller and smaller, the requirement ofregistration accuracy and thereby the requirement of alignment accuracyalso becomes more and more strict.

In current technology, there are two methods for performingphotolithography alignment. One method is a through the lens (TTL)alignment technique: using a laser beam to light up an alignment mark ona mask plate and simultaneously imaging the alignment mark onto thesurface of a wafer through an objective lens; then moving the wafer basestation to let a reference mark on the wafer base station scan the imageof the alignment mark; in the meantime, sampling the intensity of theimage and finally reaching the correct alignment position when thedetector receives a maximum intensity. The other method is an off-axis(OA) alignment technique: first, by using an off-axis alignment systemto gauge multiple alignment marks on wafer and reference marks on areference plate located on a wafer base station, alignment between thewafer and the wafer base station is thus realized, then the referencemarks on the wafer base station is aligned with the alignment marks on amask plate so that the alignment between the mask plate and the waferbase station is also realized. As such, the relative position of themask plate with respect to the wafer is determined and alignment betweenthe mask plate and the wafer is then realized.

According to the present disclosure, at present, most of the mainstreamphotolithography facilities use grating diffraction. Grating diffractionrefers to that, when a light beam is illuminated on a grating typealignment mark on a water, the beam is then diffracted and thediffracted light carries all the information about the alignment mark.The multi-level diffracted light spread out from the grating alignmentmark from different angles. After using a spatial filter to filter outthe zeroth-level light, the interference image of the ±n levels of thediffracted light on the reference plane is collected. As the featuresize is getting smaller and smaller, an interference image of morelevels of the diffracted light on the reference plane may be collected.Further, using a corresponding reference grating to scan the image alonga certain direction and the signal simultaneously detected by aphotoelectric detector. After signal processing, the position of thealignment center is then determined. The position of the alignmentcenter may be defined in the coordinate system of the wafer basestation. Then, the position of the alignment center is aligned with thealignment marks on the mask plate to realize the alignment between themask plate and the wafer.

Grating diffraction may be used in a double exposure type doublepatterning process. Referring to FIG. 1, a first grating 11 along thex-axis and a second grating 12 along the y-axis are formed in asubstrate 1. The first grating 11 is an alignment mark for the directionalong the x-axis while the second grating is an alignment mark for thedirection along the y-axis. Referring to FIG. 2, a device layer (notshown) is formed on the substrate 1 and then a photoresist layer 2 isformed on the surface of the device layer. Both the first grating 11 andthe second grating 12 are covered by the photoresist layer 2 and cannotbe seen from the top, thus they are represented by dashed lines.Referring to FIG. 3, using grating diffraction, the first alignmentcenter x0 along, the x-axis direction is obtained based on the firstgrating 11 while the second alignment center y0 along the y-axisdirection is obtained based on the second grating 12. Further, referencemarks on a mask plate which contains a first device pattern 3 are thenaligned with the first alignment center x0 and the second alignmentcenter y0, respectively. Afterwards, the photoresist layer 2 is exposedfor the first time to define the first device pattern 3.The first devicepattern 3 includes a number of parallel and equally spaced first striplines 31.

Referring to FIG. 4, reference marks on a mask plate which contains asecond device pattern 4 are aligned with the first alignment center x0and the second alignment center y0, respectively. The second devicepattern 4 includes a number of second strip lines 42 parallel to thefirst strip lines 31. Every neighboring pair of first strip lines 31correspond to a second strip line 42, and all the first strip lines 31and the second strip lines 42 are arranged in a staggered way and arespaced equally. Then the photoresist layer 2 is exposed for the secondtime to define the first device pattern 4. Finally, the photoresistlayer 2 is developed, and then the developed photoresist layer 2 is usedas a mask to etch the device layer and thus form a semiconductorstructure. The ultimately formed semiconductor structure includes anumber of the first strip lines 31 and a number of the second striplines 42.

However, referring to FIG. 3 and FIG. 4, during, the first exposureprocess, the position of the first device pattern 3, with respect to thefirst alignment center x0, may have an overlay shift. Correspondinglyduring the second exposure process, the position of the second devicepattern 4, with respect to the second alignment center y0, may also havean overlay shift. Therefore, as shown in FIG. 4, the distances betweeneach second strip line 42 and the neighboring two first strip lines 31of the second strip line 42 may not be the same, i.e. w1≠w2. As such, onone hand, the second device pattern 4 may have an overlay shift withrespect to the first alignment center x0, thus the actual position ofthe second device pattern 4 on the substrate may also have an alignmenterror with respect to its intended position; on the other hand, due tothe two overlay shifts, precise alignment between the first strip lines31 and the second strip lines 42 may not be able to achieve, thusreducing the registration accuracy between the second strip lines 42 andthe first strip lines 31. All of the above factors further affectsubsequent semiconductor fabrication processes and the performance ofthe semiconductor structure containing the second strip lines 42 and thefirst strip lines 31.

In view of the above problems, the present disclosure provides a newalignment strategy to reduce the alignment error and improve theperformance of semiconductor structures formed by a double exposure typedouble patterning process using grating diffraction.

BRIEF SUMMARY OF THE DISCLOSURE

The present disclosure includes a method for forming a photolithographyalignment mark structure. The method includes providing a semiconductorsubstrate; forming a first grating, a second grating, a third gratingand a fourth grating in the substrate; and forming a photoresist layeron a surface of the substrate. The method also include obtaining a firstalignment center along a first direction and a second alignment centeralong a second direction based on the first grating and the fourthgrating, respectively, by using grating diffraction, providing a maskplate having a fifth grating pattern and a sixth grating pattern on themask plate; aligning the mask plate with the substrate by using thefirst alignment center as an alignment center along the first directionand the second alignment center as an alignment center along the seconddirection. The method further includes reproducing the fifth gratingpattern and the sixth grating pattern in the photoresist layer on thesubstrate through an exposure process; and forming a fifth grating and asixth grating on the surface of the substrate by removing the portion ofdenatured photoresist layer.

The present disclosure also includes a photolithography alignment markstructure. The photolithography alignment mark structure includes asubstrate and a first grating, a second grating, a third grating, and afourth grating formed in the substrate. The photolithography alignmentmark structure also includes a fifth grating and a sixth grating formedon a surface of the substrate. The first grating, the second grating,and the third grating formed in the substrate are along a firstdirection; the fourth grating formed in the substrate is along a seconddirection; the first direction and the second direction areperpendicular to each other; the fifth grating and the sixth gratingformed on the mask plate are along the first direction; the gratingconstant of the first grating is smaller than the grating constant ofthe second grating; and the second grating, the third grating, the fifthgrating, and the sixth grating, have a same grating constant.

The present disclosure also includes a method for fabricatingsemiconductor structures using a photolithography alignment markstructure. The method includes providing a semiconductor substratehaving the photolithography alignment mark structure; forming a devicelayer on the surface of the substrate to cover the substrate and thephotolithography alignment mark structure; and forming a photoresistlayer on the surface of the device layer. The method also includes usinggrating diffraction to obtain a first alignment center x0 along a firstdirection based on a first grating, a third alignment, center x1 alongthe first direction based on a second grating and a fifth grating, and afourth alignment center x2 along the first direction based on a thirdgrating and a sixth grating; and performing a first exposure to define afirst device pattern in the photoresist layer by using the firstalignment center x0 as an alignment center along the first direction forthe alignment prior to the first exposure process. The first devicepattern includes a number of parallel first strip lines along the firstdirection. The method further includes performing a second exposure todefine a second device pattern in the photoresist layer by usingx′=((x1+x2)/2+x0)/2 as the alignment center along the first directionfor the alignment prior to the second exposure process, where the seconddevice pattern includes a number of parallel second strip lines alongthe first direction, and the second strip lines are interlaced with thefirst strip lines.

Other aspects of the present disclosure can be understood by thoseskilled in the art in light of the description, the claims, and thedrawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1-4 illustrate schematic top views of semiconductor structurescorresponding to certain stages of a current double exposure type doublepatterning process;

FIGS. 5-14 illustrate schematic views of photolithography alignment markstructures corresponding to certain stages of an exemplary fabricationprocess consistent with the disclosed embodiments; and

FIG. 15 illustrates an exemplary fabrication process of a lithographyalignment mark structure in one embodiment of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of theinvention, which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Referring to FIG. 1-4, a detailed description of a representative doubleexposure type double patterning process of the prior an has beenprovided in above background section. In the existing double exposuretype double patterning process, an overlay shift after each exposureprocess may be unavoidable and the two overlay shifts may further affectsubsequent semiconductor fabrication processes and the performance ofthe ultimately formed semiconductor structure.

In view of the problems described above, the disclosed embodimentsprovide a method to form a photolithography alignment mark structure fora double exposure type double patterning process, improving thealignment accuracy and the performance of the later-formed semiconductorstructure.

FIG. 15 illustrates an exemplary fabrication process of a lithographyalignment of mark structure in one embodiment of the present disclosure.As shown in FIG. 15, at the beginning of the fabrication process for thephotolithography alignment mark structure, a substrate 10 is provided(S1502). FIG. 5 shows a top view of the substrate.

In the one embodiment, the substrate 10 is made of silicon. In certainother embodiments, the substrate 10 may also be made of any otherappropriate materials, such as germanium, silicon germanium, silicon oninsulator (SIO), or germanium on insulator (GOI), etc.

In one embodiment, the substrate 10 is placed on a base station or awafer stage. The base station has an x-y coordinate system. The x-ycoordinates are used to mark the position of alignment center. A firstdirection ‘A’ is defined as along the x-axis while a second direction‘B’ is defined as along the y-axis. In addition, the base station alsohas a reference mark. The reference mark is used to locate the positionof the substrate 10 so that the position of the substrate 10 on the basestation may be determined. In the following, description of an overlayshift occurred along the first direction ‘A’ during a photolithographyprocess sets an example to illustrate the technical scheme of thepresent embodiment.

Referring to FIG. 5, a first grating 11, a second grating 12, and athird grating 13 are formed in the substrate 10 along the firstdirection ‘A’ (S1504). The grating constant of the first grating 11 issmaller than the grating constant of the second grating 12 while thegrating constant of the second grating 12 is the same as the gratingconstant of the third grating 13. The grating constant of a givengrating is the distance between any two neighboring reticles of thegrating. The second grating 12 and the third grating 13 are spaced alongthe first direction ‘A’ and are arranged parallel to each other. Thefirst grating 11 is spaced away from the second grating 12 and the thirdgrating 13 along the second direction ‘B’. A fourth grating 14, a sevengrating 17, and an eighth grating 18 are formed in the substrate 10along the second direction ‘B’. The grating constant of the fourthgrating 14 is smaller than the grating constant of the seventh grating17 while the grating constant of the seventh grating 17 is the same asthe grating constant of the eighth grating 18. The seventh grating 17and the eighth grating 18 are spaced along the second direction ‘B’ andparallel to each other. The fourth grating 14 is spaced away from theseventh grating 17 and the eighth grating 18 along the first direction‘A’.

In any other embodiments, the relative position of the first grating 11,the second grating 12, and the third grating 13 may not be limited tothe situation in the present embodiment and all the three gratings maybe spaced either along the first direction ‘A’ or along the seconddirection ‘B’. Correspondingly, the relative position of the fourthgrating, the seventh grating, and the eighth grating may not be limitedto the situation in the present embodiment and all the three ma bespaced either along the first direction ‘A’ or along the seconddirection ‘B’.

In one embodiment, the first grating 11, the second grating 12, thethird grating 13, the fourth grating 14, the seventh grating 17, and theeighth grating 18 are all scribe grooves and the scribe grooves may beformed by any appropriate techniques, such as mechanical scribing,holographic photolithography, e-beam lithography, laser interferencelithography, focused ion-beam lithography, etc.

When a holographic photolithography technique is used, the fabricationprocess starts with coating the substrate with a layer of photoresist.After baking the photoresist, the substrate is placed into aninterference optical system. Then an exposure process is performed.During the process, light waves passing through a mask plate (objectwave) interference with a parallel light beam (reference beam) and theexposure leads to an interference fringe recorded on the photoresistlayer. A portion of the photoresist layer exposed to interference lightswith a relatively high intensity is denatured. After development, theportion of the denatured photoresist is then removed. Further, scribegrooves are formed by etching the substrate 10 and the scribe groovesare the reticles of the corresponding grating. Finally, the rest of thephotoresist layer is removed.

When laser interference lithography technique is used, the laserinterferometry uses the characteristics of optical interference anddiffraction, and controls the distribution of light intensity in aninterference field through a certain combination of light beams. Thedistribution of light intensity is then recorded by using aphotosensitive material. A portion of the photoresist layer exposed tointerference light with a relatively high intensity is denatured andthus a photolithography pattern is obtained. The pattern is thenreproduced onto the substrate 10.

The scribe grooves may be formed by other techniques. For example, whenan e-beam lithography or focused ion-beam lithography technique is used,electron beam e-beam) or focused ion-beam bombardment may be used todenature the property of a portion of the photoresist layer and thusdefine the lithography pattern.

In one embodiment, the grating constant of the second grating 12 isidentical to that of the third grating 13. If the two grating constantsare not the same, when forming a fifth grating corresponding to thesecond grating 12 and a sixth grating 16 corresponding to grating 13 ina subsequent process, the difference in the grating constant may be anobstructive factor during the formation of the fifth grating and thesixth grating. Specifically, the difference in the grating constant maycause the overlay shift of the fifth grating along the first direction‘A’ with respect to the second grating 12 not equal to the overlay shiftof the sixth grating along the first direction ‘A’ with respect to thethird grating 13, thus leading to an inaccurate registration precisioncorrection. Correspondingly, similar situation also applies to theseventh grating 17 and the eighth grating 18.

In one embodiment, the grating constant of the first grating 11 is thesame as the grating constant of the fourth grating 14. However, theidentical grating constant of the first grating 11 and the fourthgrating 14 should not limit the scope of the present disclosure. Inother embodiments, the grating constant of the first grating 11 may notbe the same as the grating constant of the fourth grating.Correspondingly, whether the grating constant of the second grating isthe same as the grating constant of the seventh grating 17 or not shouldnot limit the scope of the present disclosure.

Referring to FIG. 5, the second grating 12 and the third grating 13 arearranged parallel to each other along the first direction ‘A’. Thespacing between the two gratings, w0, may be less than or equal to 100μm. The spacing w0 refers to the distance between the two closestreticles with one from the second grating 12 and the other from thethird grating 13 along the first direction ‘A’. If the spacing betweenthe second grating 12 and the third grating 13 is larger than 100 μm, arelatively large alignment error and a relatively large registrationerror may be induced during the formation of the fifth grating and thesixth grating, thus affecting the precision in adjusting theregistration accuracy in the present embodiment. Correspondingly, thesecond grating 17 and the eighth grating 18 are arranged parallel toeach other along the second direction B. The spacing between the twogratings is not greater than 100 μm. Further, the length relationbetween w0 and w′ is not limited, w0 and w′ may or may not have the samelength.

Returning to FIG. 15, after the formation of the gratings in thesubstrate 10 described above, a photoresist layer 30 is formed on thesubstrate 10 (S1506). FIG. 6 shows a to view of the structure. Thephotoresist layer 30 may be formed using any appropriate method.

Referring to FIG. 6, the photoresist layer 30 may be formed on thesubstrate 11 via a spin-coating process and may cover the substrate 11,the first grating 11, the second grating 12, the third grating 13, thefourth grating 14, the seventh grating 17, and the eighth grating 18.Moreover, because the first grating 11, the second grating 12, the thirdgrating 13, the fourth grating 14, the seventh grating 17, and theeighth grating 18 are covered by the photoresist layer 30, the gratingscannot be seen in the top view, thus they are shown by dashed lines inthe figure.

Referring to FIG. 15, after forming the photoresist layer 30, a firstalignment center x0 is obtained using grating diffraction based on thefirst grating 11 along the first direction ‘A’, while a second alignmentcenter y0 is also obtained using grating diffraction based on the fourthgrating 14 along the second direction ‘B’ (S1508). FIG. 6 schematicallyindicates the positions of the two alignment centers x0 and y0.

Referring to FIG. 6, the alignment center x0 corresponds to a point onthe x-axis of the x-y coordinates of the base station and the alignmentcenter y0 corresponds to a point on the y-axis of the x-y coordinates ofthe base station.

As an example, a detailed description on determining x0 is now given toillustrate the process of locating an alignment center by using gratingdiffraction. First, a light beam is used to illuminate the first grating11. The illumination light beam ma be a laser beam. Diffraction thenoccurs when the light beam passes through the first grating 11 and thediffracted light carries all the information about the first grating 11.The multi-level diffracted light spread out from the first grating 11from different angles and an interference image is then formed on thereference plane by collecting the multi-level diffracted light through aspatial filter. A reference grating is placed symmetrically on thereference plane with respect to the center of the main optical axis ofthe illumination light beam. The reference grating and the first grating11 have a same grating period. A corresponding probe optical fiber isplaced behind the reference grating. The probe optical fiber guides theintensity signal of the light passing through the reference grating to aphotoelectric conversion device. The photoelectric conversion deviceconverts and processes the intensity signal of the light.

Referring to FIG. 7, based on the principle of Fourier optics, asinusoidal signal corresponding to the intensity signal of the lightwith a certain period is generated in the detector. The period of thesinusoidal signal corresponds to the grating period, of the firstgrating 11 and the center of the sinusoidal signal is thus the positionof the first alignment center x0. Using a similar method, anothersinusoidal signal corresponding to the fourth grating 14 may beobtained. The second alignment center y0 corresponding to the fourthgrating 14 may then be determined based on the sinusoidal signal.

Referring to FIG. 15, further, a mask plate 21 is provided (S1510). FIG.8 shows a schematic top view of the mask plate 21. Referring to FIG. 8,the mask plate has a fifth grating pattern 15′, a sixth grating pattern16′, and a first reference mark x0′ that corresponds to the firstalignment center x0 and a ninth grating pattern 19′, a tenth gratingpattern 20′, and a second reference mark y0′ that corresponds to thesecond alignment center y0.

The mask plate 21 is used to define intended positions on the substrate10 for the grating patterns on the mask plate 21. FIG. 9 shows aschematic view of the predesigned positional relationships between thefifth grating pattern 15′ and the second grating 12 and between thesixth grating pattern 16′ and the third grating 13.

According to the predesign shown in FIG. 9, the fifth grating pattern15′ has the same grating constant as the second grating 12 and the fifthgrating pattern 15′ is stacked against the second grating 12. That isthe reticles of the fifth grating pattern 15′ are interlaced with thereticles of the second grating 12. The fifth grating pattern 15′ has anoffset of a first distance dx along the first direction ‘A’ with respectto the second grating 12. That is, the center axis of a reticle of thefifth grating pattern 15′ between two neighboring reticles of the secondgrating 12 has an offset of the first distance dx along the firstdirection ‘A’ with respect to the center line of the two neighboringreticles on the second grating 12.

Referring to FIG. 9, the sixth grating pattern 16′ has the same gratingconstant as the third grating 13 and the sixth grating pattern 16′ isstacked against the third grating 13. That is, the reticles of the sixthgrating pattern 16′ are interlaced with the reticles of the thirdgrating 13. The sixth grating pattern 16′ has an offset of a firstdistance dx along a third direction ‘C’ with respect to the thirdgrating 13. The third direction ‘C’ is the opposite direction of thefirst direction ‘A’. Therefore, the center axis of a reticle of thesixth grating pattern 16′ between two neighboring reticles of the thirdgrating 13 has an offset of the first distance dx along the thirddirection ‘C’ with respect to the center line of the two neighboringreticles on the third grating 13.

Accordingly, FIG. 10 shows a schematic view of the predesigned positionrelationships between the ninth grating pattern 19′and the seventhgrating 17 and between the tenth grating pattern 20′ and the eighthgrating 18.

According to the predesign shown in FIG. 10, the ninth grating pattern19′ has the same grating constant as the seventh grating 17 and theninth grating pattern 19′ is stacked against the seventh grating 17.That is, the reticles of the ninth grating pattern 19′ are interlacedwith the reticles of the seventh grating 17. The ninth grating pattern19′ has an offset of a second distance dy along the second direction ‘B’with respect to the seventh grating 17. That is, the center axis of areticle of the ninth grating pattern 19′ between two neighboringreticles of the seventh grating 17 has an offset of the second distancedy along the second direction ‘B’ with respect to the center line of thetwo neighboring reticles on the seventh grating 17.

Referring to FIG. 10, the tenth grating pattern 20′ has the same gratingconstant as the eighth grating 18 and the tenth grating pattern 20′ isstacked against the eighth grating 18. That is, the reticles of thetenth grating pattern 20′ are interlaced with the reticles of the eighthgrating 18. The tenth grating pattern 20′ has an offset of a seconddistance dy along a fourth direction ‘D’ with respect to the eighthgrating 18. The fourth direction is the opposite direction of the seconddirection ‘B’. Therefore, the center axis of a reticle of the tenthgrating pattern 20′ between two neighboring reticles of the eighthgrating 18 has an offset of the second distance dy along the fourthdirection ‘D’ with respect to the center line of the two neighboringreticles on the eighth grating 18.

In one embodiment, the first distance dx may be predefined. Therefore,in a subsequent lithography process, the overlay shift information of afifth grating along the first direction ‘A’ with respect to the secondgrating 12 and the overlay shift information of a sixth grating alongthe first direction ‘A’ with respect to the third grating 13 may also beincreased. This makes the actual offset of the fifth grating withrespect to the second grating 12 and the actual offset of the sixthgrating with respect to the third grating 13 measurable. The firstdistance dx may be relatively small. For example, the first distance dxmay be approximately in a range of 1 nm˜10 nm. Therefore, overlay shiftsof the fifth grating and the sixth grating may take place during asubsequent lithography process and the values of the overlay shiftsdepend linearly on dx, allowing linear addition or subtraction beperformed on the offset of the alignment center in subsequent processes,thus ensuring the implementation of the method of the presentembodiment.

Correspondingly, the second distance dy may also be predefined. Thus, ina subsequent lithography process, the overlay shift information of aninth grating along the second direction ‘B’ with respect to the seventhgrating 17 and the overlay shift information of a ninth grating alonethe first direction ‘A’ with respect to the fluid grating 13 may also beincreased.

Also referring to FIG. 15, further, an alignment process for the maskplate 21 is then performed on the top of the photoresist layer 30(S1512). Specifically, the first reference mark x0′ on the mask plate 21is aligned with the first alignment center x0 while the second referencemark y0′ on the mask plate 21 is aligned with the second alignmentcenter y0, thus the position of the mask plate 21 with respect to thebase station is then determined. The alignment lets the fifth gratingpattern 15′ be aligned with the second grating 12 along a directionperpendicular to the top surface of the photoresist layer 30 and thesixth grating pattern 16′ be aligned with the third grating 13 along thedirection perpendicular to the top surface of the photoresist layer 30.The alignment also simultaneously lets the ninth grating pattern 19 bealigned with the seventh grating 17 along the direction perpendicular tothe top surface of the photoresist layer 30 and tenth grating pattern20′ be aligned with the eighth grating 18 along the directionperpendicular to the top surface of the photoresist layer 30.

Further, referring to FIG. 15, an exposure process is performed afterthe mask plate 21 is aligned (S1514). FIG. 11 shows a schematic top viewof the structure of the substrate after the exposure process. Referringto FIG. 11, the fifth grating pattern 15′ is reproduced in thephotoresist layer 30 to define a fifth grating 15, the sixth gratingpattern 16′ is reproduced in the photoresist layer 30 to define a sixthgrating 16, the ninth grating pattern 19′ is reproduced in thephotoresist layer 30 to define a ninth grating 19, and the tenth gratingpattern 20′ is reproduced in the photoresist layer 30 to define a tenthgrating 20. Due to the influence of the lithography equipment and otherfactors, the fifth grating 15 with respect to the second grating 12 andthe sixth grating with respect to the third grating 13 may have anoverlay shift along the first direction ‘A’, or equivalently along thethird direction ‘C’. In one embodiment, the direction of the overlayshift is along the third direction ‘C’. During the photolithographyprocess, the fifth grating 15 with respect to the second grating 12 andthe sixth grating 16 with respect to the third grating 13 may have anadditional overlay shift Δx (not shown) along the first direction ‘A’corresponding to the focus depth of the photolithography apparatus.

Accordingly, the ninth grating 19 with respect to the seventh grating 17and the tenth grating with respect to the eighth grating 18 may have anoverlay shift along the second direction ‘B’, or equivalently along thefourth direction ‘D’. In one embodiment, the direction of the overlayshift is along the fourth direction ‘D’.

Returning back to FIG. 9, because the predefined fifth grating pattern15′ has an onset along the first direction ‘A’ with respect to thesecond grating 12 while the sixth grating pattern has an offset alongthe third direction ‘C’ with respect to the third grating 13. Therefore,after exposure, referring to FIG. 11, the fifth grating 15 has an offsetalong the first direction ‘A’ with respect to the second grating 12while the sixth grating 16 has an offset along the third direction ‘C’with respect to the third grating 13. Correspondingly, the ninth grating19 has an offset along the second direction ‘B’ with respect to theseventh grating 17 while the tenth grating 20 has an offset along thefourth direction ‘D’ with respect to the eighth grating 18. Also, boththe fifth grating 15 with respect to the second grating 12 and the sixthgrating 1 with respect to the third grating 13 may have another overlayshift Δy (not shown) along the second direction ‘B’ corresponding to thefocus depth of the photolithography apparatus.

Finally, also referring to FIG. 15, a development process is performedto remove the denatured portion of photoresist layer (S1516). After thedevelopment process, the photoresist layer except for the fifth grating15, the sixth grating 16, the ninth grating 19, and the tenth grating 20is removed and the surface of the substrate 10 is exposed. FIG. 12 showsa schematic view of the structure after the development process.

Accordingly, the photolithography alignment mark structure includes thefirst grating 11, the second grating 12, the third grating 13, thefourth grating 14, the seventh grating 17, and the eighth grating 18formed in the substrate 10 and the fifth grating 15, the sixth grating16, the ninth grating 19, and the tenth grating 20 formed on the surfaceof the substrate 10.

Referring to FIG. 13, with the photolithography alignment markstructure, a third alignment center x1 of a grating that consists of thesecond grating 12 and the fifth grating 15 and a fourth alignment centerx2 of a grating that consists of the third grating 13 and the sixthgrating 16 may be obtained by using grating, diffraction. The referencegrating used in the process includes a first segment corresponding tothe first grating 11, a second segment corresponding to the gratingformed by the second grating 12 and the fifth grating 15, and a thirdsegment corresponding to the grating formed by the third grating 13 andthe sixth grating 16. A probe optical fiber is placed behind of each ofthe segments to collect the intensity signal of the light passingthrough the reference gratings.

Correspondingly, refining to FIG. 13, a fifth alignment center yl of agrating that consists of the seventh grating 17 and the ninth grating 19and a sixth alignment center y2 of a grating that consists of the eighthgrating 18 and the tenth grating 20 may be obtained by using gratingdiffraction. The reference grating used in the process includes a firstsegment corresponding to the fourth grating 14, a second segmentcorresponding to the grating formed by the seventh grating 17 and theninth grating 19, and a third segment corresponding to the gratingformed by the eighth grating 18 and the tenth grating 20. A probeoptical fiber is placed behind of each of the segments to collect theintensity signal of the light passing through the reference gratings.

Returning, back to FIG. 7, because of the first distance dx and theoverlay shift Δx, the third alignment center x1 has an offset withrespect to the first alignment center x0 while the fourth alignmentcenter x2 also has an offset with respect to the first alignment centerx0. In addition, because the first distance dx is relatively small, theoverlay shift Δx and the first distance dx have a linear relationship,the offset of the third alignment center x1 with respect to the firstalignment center x0 corresponds to but is not equal to −(dx+Δx) whilethe offset of the fourth alignment center x2 with respect to the firstalignment center x0 corresponds to but is not equal to dx+Δx. The minussign wherein indicates that the offset is along, the third direction‘C’.

Accordingly, because the second distance dy is relatively small, theoverlay shift Δy and the second distance dy have a linear relationship,the offset of the fifth alignment center y1 with respect to the secondalignment center y0 corresponds to but is not equal to −(dy+Δy) whilethe offset of the sixth alignment center y2 with respect to the secondalignment center y0 corresponds to but is not equal to dy+Δy. The minussign wherein indicates that offset is along the fourth direction ‘D’.

In one embodiment, the first grating 11, the grating formed by thesecond grating 12 and the fifth grating 15, and the grating formed bythe third grating 13 and the sixth grating 16 are used as alignmentmarks along, the first direction ‘A’; the fourth grating 14, the gratingformed by the seventh grating 17 and the ninth grating 19, and thegrating formed by the eighth grating 18 and the tenth grating 20 areused as alignment marks along the second direction ‘B’.

The photolithography alignment mark structure of the embodiments of thepresent disclosure may then be used in a double exposure type doublepatterning process to improve the alignment accuracy. Specifically, adouble exposure type double patterning process using thephotolithography alignment mark structure disclosed in the embodimentsmay include the following steps:

First, during the first exposure, the first alignment center x0 is usedas the alignment center along the first direction ‘A’ and the secondalignment center y0 is used as the alignment center along the seconddirection ‘B’. After the first exposure, the first device pattern has analignment offset with respect to the intended position on the substrate.

Further, the alignment center along the first direction ‘A’ for thesecond exposure may be adjusted based on the third alignment center x1and the fourth alignment center x2. Referring to FIG. 14, because of thelinear relationship between the first distance dx and the overlay shiftΔx, linear operation may be performed by using the third alignmentcenter x1 and the fourth alignment center x2: first, the center positionbetween the third alignment center x1 and the fourth alignment center x2may be calculated and the result is x″=(x1+x2)/2. The value of x″ isregarded as the actual offset value of the first device pattern withrespect to the intended position on the substrate after the firstexposure; then, based on the offset value of the first device patternwith respect to the substrate after the first exposure, an average valueof the offset of x″ with respect to the first alignment center x0 may becalculated and the average value is (x″−x0)/2; then, an alignment centerx′=x0+(x″−x0)/2=(x″+x0)/2=((x1+x2)/2+x0)/2 may be used for the secondexposure. That is, during the second exposure process, the firstalignment center x0 is no longer used as the alignment center; instead,the adjusted position x′ is used as the alignment center.

By choosing the center position x′ between x″ and x0 as the newalignment center, after the second exposure, the alignment error of theactual position of the second device pattern along the first direction‘A’ with respect to the intended position of the second device patternon the substrate may be reduced. In the meantime, the registration errorbetween the second device pattern and the first device pattern may alsobe reduced. Thus, the alignment error between the intended position ofthe second device pattern and the actual position formed on thesubstrate after the second exposure may be reduced due to compensation,and the registration offset value of the second device pattern withrespect to the first device pattern may also be reduced. Therefore, theregistration accuracy of the second device pattern with respect to thefirst device pattern may be greatly improved, e.g., about 40%. Theimprovement may not only ensure that subsequent semiconductormanufacturing processes can be normally performed but also ensure thatthe semiconductor structure containing the second device pattern and thefirst device pattern has good performance.

In addition, during the second exposure, the alignment center of thesecond exposure along the second direction ‘B’ may also be adjustedbased on the fifth alignment center y1 and the sixth alignment centery2. Referring to the above description, the alignment center of thesecond exposure along, the second direction ‘B’ after the adjustment isy′=((y1+y2)/2+y0)/2. Using the adjusted alignment center y′, after thesecond exposure, the alignment error between the actual position of thesecond device pattern and the intended position of the second devicepattern on the substrate along the second direction ‘B’ may be reduced.

The above detailed descriptions only illustrate certain exemplaryembodiments of the present invention, and are not intended to limit thescope of the present invention. Those skilled in the art can understandthe specification as whole and technical features in the variousembodiments can be combined into other embodiments understandable tothose persons of ordinary skill in the art. Any equivalent ormodification thereof, without departing from the spirit and principle ofthe present invention, falls within the true scope of the presentinvention.

What is claimed is:
 1. A method for fabricating a photolithographyalignment mark structure, comprising: providing a semiconductorsubstrate; forming a first grating, a second grating, a third gratingand a fourth grating in the substrate; forming a photoresist layer on asurface of the substrate; obtaining a first alignment center along afirst direction and a second alignment center along a second directionbased on the first grating and the fourth grating, respectively, byusing grating diffraction; providing a mask plate having a fifth gratingpattern and a sixth grating pattern on the mask plate; aligning the maskplate with the substrate by using the first alignment center as analignment center along the first direction and the second alignmentcenter as an alignment center along the second direction; reproducingthe fifth grating pattern and the sixth grating pattern in thephotoresist layer on the substrate through an exposure process; andforming a fifth grating and a sixth grating on the surface of thesubstrate by removing a portion of photoresist layer.
 2. The methodaccording to claim 1, wherein: the first grating, the second grating,the third grating, the fourth grating, the fifth grating, and the sixthgrating together are used as the photolithography alignment markstructure.
 3. The method according to claim 2, wherein: the firstgrating, the second grating, and the third grating in the substrate areformed along the first direction; the fourth grating in the substrate isformed along the second direction; the first direction and the seconddirection are perpendicular to each other; the fifth grating pattern andthe sixth grating pattern on the mask plate are formed along the firstdirection; the grating constant of the first grating is smaller than thegrating constant of the second grating; and the second grating, thethird grating, the fifth grating pattern, and the sixth grating patternhave a same grating constant.
 4. The method according to claim 3,wherein after the mask plate is aligned with the substrate: reticles ofthe second grating are interlaced with reticles of the fifth gratingpattern, and the fifth grating pattern has an offset of a first distancealong the first direction with respect to the second grating; andreticles of the third grating are interlaced with reticles of the sixthgrating pattern, and the sixth grating pattern has an offset of thefirst distance along a reverse direction of the first direction withrespect to the third grating.
 5. The method according to claim 4,wherein: during the formation of the first grating, the second grating,the third grating, and the fourth grating in the substrate, a seventhgrating and an eighth grating are also formed in the substrate along thesecond direction; the mask plate also has a ninth grating pattern and atenth grating pattern along the second direction; after the mask plateis aligned with the substrate, reticles of the seventh grating areinterlaced with reticles of the ninth grating pattern, and the ninthgrating pattern has an offset of a second distance along the seconddirection with respect to the seventh grating; and reticles of theeighth grating are interlaced with reticles of the tenth gratingpattern, and the tenth grating pattern has an offset of the seconddistance along a reverse direction of the second direction with respectto the eighth grating.
 6. The method according to claim 5, wherein:during an exposure process, the ninth grating pattern and the tenthgrating pattern on the mask plate are also reproduced in the photoresistlayer on the substrate simultaneously with the fifth grating pattern andthe sixth grating pattern; during formation of the fifth grating and thesixth grating on the substrate, a ninth grating and a tenth grating arealso formed on the substrate along the second direction; and thephotolithography alignment mark structure also includes the seventhgrating, the eighth grating, the ninth grating, and the tenth grating.7. The method according to claim 1 wherein the substrate may be made ofany appropriate semiconductor materials including silicon, germanium,silicon germanium, silicon on insulator (SIO), or germanium on insulator(GOI), etc.
 8. The method according to claim 4, wherein the firstdistance is in a range of 1 nm˜10 nm.
 9. The method according to claim4, wherein the second grating and the third grating are arrangedparallel to each other along the first direction and a distance betweenthe second grating and the third grating is not greater than 100 μm. 10.A photolithography alignment mark structure, comprising: a substrate; afirst grating, a second grating, a third grating, and a fourth gratingformed in the substrate; a fifth grating and a sixth grating formed on asurface of the substrate, wherein; the first grating, the secondgrating, and the third grating formed in the substrate are along a firstdirection; the fourth grating formed in the substrate is along a seconddirection; the first direction and the second direction areperpendicular to each other; the fifth grating and the sixth gratingformed on the mask plate are along the first direction; the gratingconstant of the first grating is smaller than the grating constant ofthe second grating; and the second grating, the third grating, the fifthgrating, and the sixth grating have a same grating constant.
 11. Thephotolithography alignment mark structure according to claim 10,wherein: the first grating, the second grating, the third grating, andthe fourth grating are all scribe grooves.
 12. The photolithographyalignment mark structure according to claim 10, wherein: reticles of thesecond grating are interlaced with reticles of the fifth grating and thefifth grating has an offset of a first distance along the firstdirection with respect to the second grating; reticles of the thirdgrating are interlaced with reticles of the sixth grating and the sixthgrating has an offset of a first distance along a reverse direction ofthe first direction with respect to the second grating; and the fifthgrating with respect to the second grating and the sixth grating withrespect to the third grating have an Overlay shift along the firstdirection.
 13. The photolithography alignment mark structure accordingto claim 12, wherein the alignment mark structure also includes: aseventh grating and an eighth grating formed in the substrate along thesecond direction, wherein the seventh grating and the eighth grating areboth scribe grooves; and a ninth grating and a tenth grating formed onthe substrate along the second direction, wherein the grating constantof the fourth grating is smaller than the grating constant of theseventh grating; and the seventh grating, the eighth grating, the ninthgrating pattern, and the tenth grating have a same grating constant. 14.The photolithography alignment mark structure according to claim 13,wherein: reticles of the seventh grating are interlaced with reticles ofthe ninth grating and the ninth grating has an offset of the seconddistance along the second direction with respect to the seventh grating;reticles of the eighth grating are interlaced with reticles of the tenthgrating and the tenth grating has an offset of the second distance alonga reverse direction of the second direction with respect to the eighthgrating; and the ninth grating with respect to the seventh grating andthe tenth grating with respect to the eighth grating have an overlayshift along the second direction.
 15. The photolithography alignmentmark structure according to claim 10, wherein the second grating and thethird grating are parallel to each other along the first direction and adistance between the second grating and the third grating is not greaterthan 100 μm.
 16. A method for fabricating semiconductor structures usinga photolithography alignment mark structure, comprising: providing asemiconductor substrate having the photolithography alignment markstructure; forming a device layer on the surface of the substrate tocover the substrate and the photolithography alignment mark structure;forming a photoresist layer on the surface of the device layer; usinggrating diffraction to obtain a first alignment center x0 along a firstdirection based on a first grating, a third alignment center x1 alongthe first direction based on a second grating and a fifth grating, and afourth alignment center x2 along the first direction based on a thirdgrating and a sixth grating; performing a first exposure to define afirst device pattern in the photoresist layer by using the firstalignment center x0 as an alignment, center along the first directionfor the alignment prior to the first exposure process, wherein the firstdevice pattern includes a number of parallel first strip lines along thefirst direction; and performing a second exposure to define a seconddevice pattern in the photoresist layer by using a pointx′=((x1+x2)/2+x0)/2 as the alignment center along the first directionfor the alignment prior to the second exposure process, wherein thesecond device pattern includes a number of parallel second strip linesalong the first direction, and the second strip lines are interlacedwith the first strip lines.
 17. The method according to claim 16,further including: removing the photoresist layer except for the firstdevice pattern and the second device pattern to form a patternedphotolithography layer; and using the patterned photolithography layeras a mask to etch the device layer until a surface of the substrate isexposed.
 18. The method according to claim 16, wherein a secondalignment center y0 is obtained based on the fourth grating usinggrating diffraction, wherein the second alignment center y0 is used asan alignment center along the second direction for both the firstexposure process and the second exposure process.
 19. The methodaccording to claim 16, wherein the photolithography alignment markstructure also includes a seventh grating and an eighth grating in thesubstrate along the second direction and a ninth grating and a tenthgrating on the substrate along the second direction, and the methodfurther includes: prior to the first exposure process, by using gratingdiffraction, obtaining a second alignment center y0 along the seconddirection based on the fourth grating, a fifth alignment center y1 alongthe second direction based on the seventh grating and the ninth grating,and a sixth alignment center y2 along the second direction based on theeighth grating and the tenth grating; the second alignment center y0 isused as an alignment center along the second direction for the alignmentprior to the first exposure; and an alignment center y′=((y1+y2)/2+y0)/2is used as an alignment center along the second direction for thealignment prior to the second exposure.